Simple DoF Calculator: Explain the math... Review

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Explain the math...I'm no genius but shouldn't you get shallower depth of field the larger the film plane?
I like the interface. But if the math is wrong what's the point.
I think I may have just wasted a buck.

Simple functional AppGreat and easy to use app, all the basic settings and values easily accessed and read, best out of the ones I have tried.
The only problem is dept of field or focus distance when below 1Metre level needs to read in millimetres. eg 500mm or 100mm not 0.50m which is awkward.
I am frequently using a Canon 100mm macro lens that can have a DoF in the mm range however this app won't display these small values which limits the usefullness of the app severely for me.